This invention relates to a method of producing conventional grain-oriented silicon steel with improved magnetic properties. More particularly, this invention relates to a method of improving cube-on-edge grain-oriented silicon steel processing by providing small but sufficient amounts of boron in the cold-rolled strip so as to improve magnetic permeability and core loss values.
In the manufacture of grain-oriented silicon steel, it is known that the Goss secondary recrystallization texture, [110][001], in accordance with Miller's Indices, results in improved magnetic properties, particularly permeability and core loss over nonoriented steels. The Goss texture refers to the body-centered cubic lattice comprising the grain or crystal being oriented in the cube-on-edge position. The texture or grain orientation of this type has a cube edge parallel to the rolling direction in the plane of rolling, with the (110) plane being in the sheet plane. As is well known, steels having this orientation are characterized by a relatively high permeability in the rolling direction and a relatively low permeability in a direction at right angles thereto.
In the manufacture of grain-oriented silicon steel, typical steps include providing a melt on the order of 2-4.5% silicon, casting the melt, such as by ingot or continuous casting processes, hot rolling the steel, cold rolling the steel to final gauge with an intermediate annealing when two or more cold-rolling stages are used, decarburizing the steel, applying a refractory oxide base coating, such as magnesium oxide coating, to the steel, and final texture annealing the steel at elevated temperatures in order to produce the desired secondary recrystallization and purification treatment to remove impurities, such as nitrogen and sulfur. The development of the cube-on-edge orientations is dependent upon the mechanism of secondary recrystallization wherein during recrystallization, secondary cube-on-edge oriented grains are preferentially grown at the expense of primary grains having a different and undesirable orientation.
Grain-oriented silicon steel is conventionally used in electrical applications, such as power transformers, distribution transformers, generators, and the like. The silicon content of the steel and electrical applications permit cyclic variation of the applied magnetic field with limited energy loss, which is termed core loss. It is desirable, therefore, in steel of this type, to reduce core loss. It is known that the core loss is made up of two main components, that due to the hysteresis effect, and that due to eddy currents. The magnitude of the eddy currents is also limited by the resistance of the path through which they flow. The resistance of the core material is determined by the resistivity of the material and its thickness or cross-sectional area. Consequently, it is desirable as shown by a trend in the industry that magnetic materials having a high resistivity be produced in thin sheets in order that eddy current losses be kept to a minimum.
Numerous attempts have been made for improving the quality of cube-on-edge grain-oriented electromagnetic silicon steels by the addition of boron to the steel melt. For example, U.S. Pat. No. 3,873,381, issued May 25, 1975, uses boron and nitrogen additions to control grain growth during the primary grain-growth stage in addition to the presence of manganese and sulfur. The reference discloses the need for large amounts of boron on the order of 20 to 120 parts per million (ppm), nitrogen on the order of 3 to 100 ppm in the steel melt. The resulting cold-rolled strip is then subject to special processing including a wet decarburizing atmosphere.
Other attempts to improve magnetic properties include the addition to the silicon-iron melt of a smaller amount of boron to the melt such that the hot-rolled band contains a small but critical amount of boron in critical proportions to the nitrogen content of the metal while controlling the manganese and sulfur to achieve high permeability silicon steels. U.S. Pat. No. 3,905,842, issued Sept. 16, 1975, discloses adding a source of boron to the melt and thereafter processing the melt to provide a cold-rolled sheet containing 5 to 45 ppm boron and from 15 to 95 ppm nitrogen and the proportions of nitrogen and boron being in the ratio of 2 to 4 parts of nitrogen to one part of boron. Sulfur may range from 0.007 to 0.06% and manganese from 0.002 to 0.1%, by weight. The steel of the reference includes at least 0.007% sulfur in solute form during final texture annealing. A similar steel is disclosed in U.S. Pat. No. 3,905,843, issued Sept. 16, 1975, wherein the ratio of nitrogen to boron ranges from 1 to 15 and the ratio of manganese to sulfur is maintained to less than 2.1. The cold-rolling schedules for the processes of both of these references includes an intermediate annealing step between the cold-rolling stages and a final heavy cold reduction on the order of greater than 70%, or 80% or more, to final gauge.
Other attempts have been made to simplify the silicon-iron sheet production process by eliminating one processing step, such as by changing a two-stage cold-rolling operation to a direct cold-rolling process. U.S. Pat. No. 3,957,546, issued May 18, 1976, discloses that when the manganese-to-sulfur ratio is less than 1.8, the hot-rolled band can be cold rolled directly to final thickness without intermediate anneals. An improvement on the direct cold-rolling process is disclosed in U.S. Pat. 4,078,952, issued Mar. 14, 1978. That reference disclosed preparing a band from a melt having 6 to 18 ppm boron and producing a hot-rolled band having a manganese-to-sulfur ratio of at least 1.83 for the purpose of providing uniformity between the poor end and the good end of coils.
Although it is known from the above-cited patents that the quality of electromagnetic silicon steel can be improved by adding controlled amounts of boron to the melt to produce so-called high permeability steels having permeabilities of at least 1870 (G/O.sub.e) at 10 oersteds and core loss of no more than 0.700 watts per pound (WPP) at 17 kilogauss, as with most all processes, they are in need of improvement. U.S. Pat. 4,000,015, issued Dec. 28, 1976, discloses a method of controlling the dew point of the hydrogen-bearing atmosphere used to decarburize boron-bearing grain-oriented silicon steels having a cube-on-edge orientation. To such steels, it has also been disclosed in U.S. Pat. No. 4,054,470, issued Oct. 18, 1977, that copper may be present in the steel melt for the purpose of inhibiting primary grain growth. U.S. Pat. No. 4,338,144, issued July 6, 1982, discloses modifying the boron-bearing composition to have less than 20 ppm solute nitrogen and a manganese-to-sulfur ratio of at least 2.1 and thereafter heating the sheet in a nitrogen-bearing hydrogen atmosphere to a temperature sufficient to effect secondary recrystallization. It is also known that large boron levels in silicon steel tend to promote brittleness and reduce the capability of welding the steel. Welding can be an important operation within the process to facilitate processing, increase yield and cut costs of manufacturing production. Although it is preferable to weld hot-rolled band prior to further processing, welding can occur at other stages of production. For example, U.S. Pat. No. 4,244,757, issued Jan. 13, 1981, discloses a method of controlling nitrogen and phosphorus, as both of those elements were found to adversely affect the weldability of the steel.
It is also known that grain-oriented silicon steels containing relatively large amounts of boron result in an increase in the secondary grain size. Typical high permeability silicon steels have grain sizes greater than 10 mm. The eddy current portion of the core loss is directly related to the size of the secondary grains. The larger the grain size, the larger the core loss. Attempts have been made, such as in U.S. Pat. 4,548,655, issued Oct. 22, 1985, to reduce watt loss by achieving fine secondary grain size in boron-bearing silicon steels during final texture annealing. Another manner of reducing core loss values is by reducing the sheet thickness. U.S. Pat. No. 4,608,100, issued Aug. 26, 1986, discloses a method of producing thin gauge oriented silicon steel.
Generally, all of the development work related to the boron-bearing steels in the above-cited patents was done on cube-on-edge grain-oriented silicon steels having a final gauge of about 10 mils or greater. Such steels rely on the high boron content for the primary grain growth inhibition for providing high permeability silicon steels. Such silicon steels also generally undergo cold reduction operations to final gauge wherein a final heavy cold reduction on the order of greater than 80% is made in order to facilitate the grain orientation.
What is needed is a method for producing conventional grain-oriented silicon steel which takes advantage of the benefits of boron additions without the disadvantages thereof. It is desirable that a method be developed for reducing the final gauge of the boron-containing steels to less than nominally 10 mils while maintaining the secondary grain size in the order of conventional grain-oriented silicon steels which do not contain boron. Furthermore, it is desirable to improve the weldability of the steel produced thereby over high permeability steels, such as in U.S. Pat. No. 3,905,842, cited above. The improved process should result in silicon-iron sheet of nominally 10 mils or less characterized by magnetic permeability of at least 1850 (G/O.sub.e) at 10 oersteds and improved core loss values over that of conventional grain-oriented silicon steels.